Unzip offline-sdk-docs for m offline-sdk-docs or m docs

offline-sdk-docs is the most useful target for local developers
verifying documentation changes.  Unzip offline-sdk-docs.zip
into out/target/common/docs/offline-sdk/ whenever the offline-sdk-docs
or docs targets are built.

Fixes: 124115897
Test: m offline-sdk-docs
Change-Id: I92ce65742e7475cc64a688f2b1893c96ecfd2a52